MARVEL Junior Seminar — December 2017

Robert Baldock (EPFL, THEOS) gives a presentation on 'Calculating Multicomponent Phase Diagrams with Nested Sampling' and Alexander Hampel (ETHZ) on 'Describing the combined structural and metal-insulator transition in rare-earth nickelates with ab-initio (DFT+U and DFT+DMFT) methods'. The seminar is facilitated by Gloria Capano.

The MARVEL Junior Seminars aim to intensify interactions between the MARVEL Junior scientists belonging to different research groups located at EPFL. The EPFL community interested in MARVEL research topics is very welcome to attend. We believe that these events will be central for establishing a vibrant community.

Each seminar consists of two presentations of 25 minutes each, allowing to present on a scientific question in depth, each presentation being followed by 10 minutes for discussion. The discussion is facilitated and timed by the chairperson of the day whose mission is to ensure active lively interactions between the audience and the speakers.

Pizza is served as of 11:45 in the MED hall (floor 0), and after the seminar at 13:30 you are cordially invited for coffee and dessert to continue discussion with the speakers.

The automated calculation of complete phase diagrams, directly from a first-principles or empirical potential energy function, is one of the outstanding challenges in computational materials science. Here, we show how nested sampling, a Bayesian Markov chain Monte Carlo algorithm, can be transformed into a powerful tool for exactly this task. Since our nested sampling algorithms do not require previous information about the nature or location of phase transitions, or the atomic structures of phases formed by the material, they can be used as agnostic, black-box tools for phase diagram calculation. In this way our approach is different from methods such as thermodynamic integration and self-consistent phonons, and closer in spirit to algorithms such as Wang-Landau sampling and parallel tempering.

I will begin this talk by reviewing the theory of nested sampling, and describe our recent advances in the efficient calculation of pressure-temperature phase diagrams. Next, I will introduce a new nested sampling algorithm, which enables the one-shot calculation of composition-temperature phase diagrams for alloys, including alloys that exhibit a miscibility gap whereby the material separates into domains of different composition. I will showcase the efficacy of the approach by presenting the binary phase diagrams of a Lennard-Jones alloy (continuous atomistic state space) and Ga_{x}In_{1-x}P as described using a lattice model (discrete atomistic state space).

Perovskite rare-earth nickelates, RNiO3, display a rich and only partially understood phase diagram, where all compounds with R from Pr to Lu undergo a metal-insulator transition (MIT) that is accompanied by a structural distortion [1]. This distortion breaks the symmetry between formerly equivalent Ni sites and can (in the simplest picture) be understood as a charge disproportionation of the Ni3+ cations into Ni2+ and Ni4+. Both the transition temperature as well as the strength of the charge disproportionation vary strongly as function of the rare-earth ion.

Here, we use density functional theory (DFT) and its extensions (DFT+U, DFT+DMFT), combined with symmetry-based distortion mode analysis to explore the interplay between lattice distortions, magnetic order, and electronic correlation effects in rare-earth nickelates. The decomposition of the structure into distortion modes allows to determine the structural ground state of the system by varying the different structural degrees of freedom individually and comparing total energies calculated within the DFT+DMFT framework. Therefore, the influence of subtle structural changes, as well as the influence of the interaction parameters (U and J) on the ground state structure can be analyzed. Thereby, we also explore the capabilities of the DFT+DMFT method to describe complex materials with coupled electronic and structural degrees of freedom, by benchmarking the accuracy of such calculations and comparing the corresponding results with DFT+U results and available experimental data.